"Originally, I'd written the script with Billie Piper [who played the Moment] coming back, because she sort of disappears in the story.

"But when we thought we could get Tom Baker, I put him in. So that could be the Moment, just deciding to talk to the Doctor through this appearance."

So what was his guess as to what it was all about?

"I do think of it as, in the distant, distant future, long after Jodie [Whittaker] and long after we're all dead, the Doctor settles down and chooses a different incarnation per day to revisit," Moffat added.

"I like that idea."

Doctor Who will return with 'Twice Upon a Time', which airs on BBC One on Christmas Day at 5.30pm.